The invention relates to the demodulation of digital signalling messages that are transmitted in cellular radio telephone systems and, in particular, to systems that use Manchester-coded digital frequency modulation (FM) for the transmission of such messages.
It is known to use Manchester coding of binary data in the transmission of data, e.g., the AMPS (Advanced Mobile Phone Service System) system in North America. Digital data is impressed upon the radio carrier frequency by means of FM and encoded before FM using Manchester (also known as split-phase) coding, in which an information bit "1" is represented by a two-bit codeword, or symbol, 10, and an information bit "0" is represented by a two-bit codeword, or symbol, 01. The transmitted codeword bit rate is twice the information bit rate.
Manchester coding offers many advantages. For example, the mean of the signal transmitted over the channel can be zero for information bits of either polarity, so that the channel does not require a true d.c. response. In a frequency modulated radio system, frequency errors between the transmitter and receiver cause an offset in the mean frequency of the received signal so that the mean level of a frequency modulating signal is not faithfully reproduced. Manchester coding minimizes such frequency errors.
In non-Manchester coding systems, a "1" and a "0" are distinguished by using a static signal level. Errors in the signal level arising in transmission can cause bit errors. In systems employing Manchester coding, the static level, or mean level, of the signal is not important to distinguish between a "1" and a "0". Further advantages of Manchester coding include a high signal transition density (frequent changes between "0" and "1" bits) and the ability to detect data errors and sequence violations since the two-bit words 00 and 11 represent errors rather than data.
One reason for systems to employ Manchester coding is to eliminate sensitivity to frequency errors, which cause the phases at the symbol end-points to systematically drift higher or lower and often not to return to the same value. Conventional FM systems employ a frequency discriminator to demodulate the Manchester-coded FM signal, an integrate-and-dump circuit, and means for inverting the second half of the symbol for combination with the first half of the symbol. Such systems are subject to frequency errors. Accordingly, it is desirable to equip a system with a means for improving tolerance to frequency errors, overcoming many of the disadvantages associated with conventional systems.